Oxidation-stabilized biodiesel

ABSTRACT

The present invention relates to oxidation-stabilized biodiesel and methods for the production thereof.

The present invention relates to oxidation-stabilized biodiesel and to methods for the production thereof.

Biodiesel or fatty acid alkyl esters, in particular fatty acid methyl esters or fatty acid ethyl esters, are used to an increasing extent as alternatives to conventional diesel fuel. It has become established as a regenerative and CO₂-neutral first generation fuel.

The production takes place predominantly by transesterification of vegetable oils (fatty add triglycerides), such as, for example, palm oil, rapeseed oil, soybean oil or sunflower oil, with methanol or ethanol to give fatty acid methyl and ethyl esters by catalysis of bases such as NaOH, KOH, sodium methylate or potassium methylate. Besides the pure fats, mixtures thereof, as well as waste fats and animal fats of highly diverse origin and composition, are also used.

In contrast to mineral oils, these fats have been subjected to more severe aging processes. The increased oxidation and the formation of free fatty acids are essentially responsible for the aging. The use of antioxidants has therefore become more and more established. Thus, for example, EP-A 1736528 discloses the use of substituted alkylphenols. These are added to the biodiesel in concentrations of from 0.005 to 0.5% by weight.

Moreover, the fatty acid oxidation is initiated and/or promoted by radical formers such as e.g. copper (I) and zinc (II). Since these metals are sometimes used in the alloys of pipelines, storage tanks and reactors of the biodiesel plants, the rate of the formation of free fatty acids is increased as a result, especially in the event of prolonged storage time and/or increased storage temperature of the fatty acid methyl esters. The literature describes, as a countermeasure, the addition of metal chelating agents such as e.g. citric acid (WO 2007/102948), without the disadvantageous effect on the aforementioned metals being compensated here. Moreover, as a result of adding acids, the acid number of the biodiesel increases, which may lead to an increased rate of corrosion and therefore does not meet the standard for Biodiesel EN 590 with an acid number of at most 0.2 mg KOH/g. A further disadvantage is the poor solubility of the citric acid in the specified solvents, meaning that firstly a presolution has to be produced. The additive known in this connection is primarily butylhydroxytoluene (BHT). This is likewise added to the biodiesel in amounts of from 0.005 to 0.5% by weight. Furthermore, the use of the dimeric compound 2,2′-methylenebis(6-tert-butyl-4-methylphenol), called bisphenol hereinbelow, is described.

Standard commercial compounds are obtainable e.g. from Lanxess Distribution GmbH under the names Baynox® EU or Baynox® plus. These solids are dissolved in biodiesel, petroleum or toluene prior to application.

Alternatively to this, the aforementioned active ingredients are already obtainable in dissolved form on the market, such as e.g. Baynox® Solution (20% by weight BHT in biodiesel) or Baynox® plus Solution 20% by weight Baynox® plus in biodiesel) from Lanxess Distribution GmbH. However, the concentration here is limited to 20% by weight of active ingredient and below 10° C. crystallization can even start, as a result of which pipelines and sensitive instruments can be adversely affected. More highly concentrated solutions have to be heated in order to exclude a crystallization.

WO 2007/102948 describes more highly concentrated solutions that are stable at room temperature and consist of up to 40% by weight of the antioxidant tert-butylhydroquinone (TBHQ). However, this substance is hazardous to water and is only soluble in a comparatively expensive solvent ethylene glycol monobutyl ether. Moreover, these products can only be produced using a cosolvent (butyl acetate) and therefore have a comparatively low flashpoint of 40-70° C., which adversely affects safety and handling.

Other liquid formulations consist either of isomer mixtures of different phenols and therefore have fluctuating product properties, comprise phenylenediamines which make the solution dark red in color, or have other disadvantages (low flashpoint, high water hazard potential, high tendency towards crystallization, high viscosity, etc.).

As suitable antioxidants, the literature also describes the substituted phenols 2,6-di-tert butylphenol, 2,4,6-tri-tert-butylphenol. However, these have a comparatively low effectiveness.

The mono- and dialkylated hydroquinones, such as e.g. 2-tert-butylhydroquinone (TBHQ) and 2,5-di-tert-butylhydroquinone are known as particularly effective antioxidants especially for polyunsaturated fatty acids, such as e.g. soybean oil (WO2009/108851). However, these compounds are sparingly soluble and have a dark color particularly at higher concentrations.

It was therefore an object of the present invention to provide oxidation-stabilized biodiesel and improved methods for the production thereof which permit the production of biodiesel with high antioxidant concentration coupled with simultaneously high effectiveness at low dosage, coupled with simple handling. The mixture used for this purpose should ideally have a flashpoint>60° C., preferably>7° C. and remain stable at temperatures down to −5° C. Furthermore, the biodiesel produced therewith should have a corrosion-inhibiting effect and protect the apparatuses used (tanks, pipelines etc.) as well as the biodiesel itself against oxidation, where even biodiesel contaminated with copper and zinc is to be stabilized effectively and without increasing the acid number.

Surprisingly, it has been found that oxidation-stabilized biodiesel can be produced by the addition of from 0.005 to 0.5% by weight of a mixture of:

A) 40-60% by weight of at least one C₁-C₄-alkylated mono- and at least one C₁-C₄-alkylated bisphenol,

B) 1-5% by weight of chelating agent and

C) 35-59% by weight of solvent, selected from the group of aprotic amides with a chain length of C₁-C₄ (C1) and/or cyclic ketals (C2) and/or dibasic esters with a chain length of C₄-C₆ (C3),

where the sum of the constituents of the mixture is 100% by weight. At the same time, however, the metal-containing apparatuses/apparatus components used (copper, brass or bronze) are also protected against corrosion.

The present invention therefore provides a method for the production of oxidation-stabilized biodiesel (fatty acid alkyl esters), according to which 0.005 to 0.5% by weight of a mixture of

A) 40-60% by weight, preferably 45-55% by weight, of at least one C₁-C₄-alkylated mono- and at least one C₁-C₄-alkylated bisphenol,

B) 1-5% by weight, particularly preferably 1.2-2.6% by weight, of chelating agent and

C) 35-59% by weight of solvent, selected from the group of aprotic amides with a chain length of C₂-C₄ (C1) and/or cyclic ketals (C2) and/or dibasic esters with a chain length of C₄-C₆ (C3),

where the sum of the constituents A), B) and C) is 100% by weight, is metered into the biodiesel, and also the oxidation-stabilized biodiesel (fatty acid alkyl esters) comprising 0.005 to 0.5% by weight of a mixture of

A) 40-60% by weight, preferably 45-55% by weight, of at least one C₁-C₄-alkylated mono- and at least one C₁-C₄-alkylated bisphenol,

B) 1-5% by weight, particularly preferably 1.2-2.6% by weight, of chelating agent for copper and zinc ions and

C) 35-59% by weight of solvent, selected from the group of the aprotic amides with a chain length of C₁-C₄ (C1) and/or cyclic ketals (C2) and/or dibasic esters with a chain length of C₄-C₆ (C3),

where the sum of the constituents A), B) and C) is 100% by weight.

The invention further provides compositions comprising

A) 40-60% by weight, preferably 45-55% by weight, of at least one C₁-C₄-alkylated mono- and at least one C₁-C₄-alkylated bisphenol,

B) 15% by weight, particularly preferably 1.2-2.6% by weight, of chelating agent for copper and zinc ions and

C) 35-59% by weight of solvent, selected from the group of the aprotic amides with a chain length of C₁-C₄ (C1) and/or cyclic ketals (C2) and/or dibasic esters with a chain length of C₄-C₆ (C3).

The aforementioned mixtures and compositions comprise here the following variants of:

A, B, C1;

A, B, C2;

A, B, C and C2;

A, B, C3;

A, B, C and C3;

A, B, C2 and C3;

A, B, C1, C2 and C3;

A, C1;

A, C2;

A, C1 and C2;

A, C3;

A, C1 and C3;

A, C2 and C3;

A, C1, C2 and C3

Preference is given to the following mixtures and compositions of:

A, B, C1;

A, B, C2;

A, B, C3;

A, C1;

A, C2;

A, C3.

Very particular preference is given to the following mixtures and compositions:

A, B, C2 and

A, C2,

where for A, B, C1, C2 and C3 have the aforementioned meaning for all specified mixtures and compositions.

In the context of the invention, biodiesel is fatty acid methyl ester (FAME) and/or a fatty acid ethyl ester (FAEE). Mixtures of FAME or FAEE are also possible. This term encompasses the esters formed by transesterification of vegetable oils (fatty acid glycerides), such as, for example, palm oils, rapeseed oil, soybean oil or sunflower oil, and/or waste fats and/or animal fats, with methanol or ethanol to give fatty acid methyl or ethyl esters by catalysis of bases, such as NaOH, KOH, sodium methylate or potassium methylate.

In the context of the invention, the combination of at least one C₁-C₄-alkylated mono- and at least one C₁-C₄-alkylated bisphenol (A) is preferably a mixture comprising 2,6-di-tert-butyl-4-methylphenol (BHT) and 2,2′-methylenebis(6tert-butyl-4-methylphenol). By combining at least one C₁-C₄-alkylated mono- and at least one C₁-C₄-alkylated bisphenol (A), i.e. the combination of antioxidants with differing effectiveness, it is possible to influence the ceiling effect in a positive manner, i.e. the flattening off of the dose-effect curve at higher active ingredient concentrations, and also the shelf life, i.e. the storability over prolonged periods (>90 days). By adding a dihydroxyphenol such as e.g. tert-butylhydroquinone (TBHQ), this effect can be yet further improved and at the same time the oxidation stability at low antioxidant contents (e.g. 100-200 ppm) can be considerably increased.

In the context of the invention, the chelating agent is aromatic triazoles. These preferably have a pH>6 in aqueous solution. These chelating agents fulfill the task of complexing copper and zinc onto metal surfaces or in the biodiesel and of preventing an acceleration of the autoxidation chain.

In a particularly preferred embodiment of the present invention, benzotriazole or tolyltriazole is used as chelating agent.

In the context of the invention, the solvents used are preferably those with a flashpoint>60° C., preferably>70° C., particularly preferably>80° C., and those with a melting point<−10° C. preferably −20° C. Furthermore, the solvents should be easy to handle and not be severely hazardous to water.

In a particularly preferred embodiment of the invention, aprotic amides with a chain length of C₂-C₄ and/or cyclic ketals and/or dibasic esters with a chain length of are used as solvent.

Here, N,N-dimethylacetamide, N,N-dimethylpropionylamide and/or N-methylpyrrolidone are particularly preferred as aprotic amides.

Here, dibasic esters with a chain length of C₄-C₆ preferably dimethyl glutarate, dimethayl succinate and/or dimethyl adipate are.

Here, preferred cyclic ketals are the reaction products of glycerol with at least one C₁- to C₆-aldehyde and/or the reaction products of glycerol with at least one C₃- to C₆-ketone, with 2,2-dimethyl-1,3-dioxolane-4-methanol being particularly preferred. For combinations of 2,6-di-tert-butyl-4-methylphenol (BHT) and 2,2′-methylenebis(6-tert-butyl-4-methylphenol (A) with tert-butylhydroquinone, on account of the poorer solubility of the tert-butylhydroquinone, mixtures of the cyclic ketals 2,2-dimethyl-1,3-dioxolane-4-methanol and 2-isobutyl-2-methyl-1,3-dioxolane-4-methanol (C2) are also particularly advantageously used in the weight ratio 3:1 to 10:1.

In a further preferred embodiment of the present invention, the ratio (weight ratio) of C₁-C₄-alkylated mono- to C₁-C₄-alkylated bisphenols, preferably 2,6-di-tert-butyl-4-methylphenol (BHT) to 2,2-methylenebis(6-tert-butyl-4-methylphenol), is 1:10 to 2:1, particularly preferably 1:5 to 1:1.

In another preferred embodiment of the present invention, the 40-60% by weight of A) additionally comprise 1.0-9% by weight, particularly preferably 3-8% by weight, of tert-butylhydroquinone.

The present invention likewise relates to a method for producing the mixture of the 1:0 constituents A), B) and C), and also a method for producing the composition comprising the constituents A), B) and C), according to which firstly the solvent C) is introduced, heated to at least 40° C. and then the constituents A) and optionally B) are metered in, preferably with stirring, and then the mixture is further stirred until a clear solution is formed.

Suitable mixing units are in principle all known stirring elements, preferably axial, radial or tangential stirring elements, as described in Wilke et al., Rührtechnik Verfahrenstechnische und apparative Grundlagen [Stirring technology, processing and apparatus principles], Dr. Alfred Hüthig Verlag Heidelberg, 2nd edition, 1991, pages 92 to 97.

Moreover, the present invention provides the use of the biodiesel stabilized by the method according to the invention as fuel, and also the oxidation-stabilized biodiesel comprising the constituents A), B) and C) in a fraction of from 0.005 to 0.5% by weight. This oxidation-stabilized biodiesel is preferably produced by stirring in from 0.005 to 0.5% by weight of the mixture comprising the constituents A), B) and C). Suitable mixing units are in principle all known stirring elements, preferably axial, radial or tangential stirring elements, as described in Wilke et al., Rührtechnik Verfahrenstechnische und apparative Grundlagen [Stirring technology, processing and apparatus principles], Dr. Alfred Hüthig Verlag Heidelberg, 2nd edition, 1991, pages 92 to 97.

As regards the other parameters, reference is made to the above statements relating to the method according to the invention.

The scope of the invention encompasses all of the radical definitions, indices, parameters and explanations listed above and below and given in general or in preferred ranges with one another, i.e. also between the respective ranges and preferred ranges in any desired combination.

The examples below aim to illustrate the invention without, however, limiting it in its scope.

WORKING EXAMPLES

The following compounds were used here:

A) 20% by weight of 2,6-di-tert-butyl-4-methylphenol (BHT) stirred into biodiesel

B) 20% by weight of 2,2′-methylenebis(6-tert-butyl-4-methylphenol) stirred into biodiesel

C) Mixture of 53% by weight of dimethylacetamide, 45% by weight of a mixture of 2,6-di-tert-butyl-4-methylphenol and 2,2′-methylenebis(6-tert-butyl-4-methylphenol) in the weight ratio 1:1 and 2% by weight of tolyltriazole.

-   -   This was prepared by the following steps: the solvent         dimethylacetamide is initially introduced and heated to at least         40° C. and then 2,6-di-tert-butyl-4-methylphenol and         2,2′-methylenebis(6-tert-butyl-4-methylphenol) were metered in         with the tolyltriazole with stirring, and the mixture was         further stirred until a clear solution was formed.

D) 30% by weight of tert-butylhydroquinone in 30% by weight of diethylene glycol monobutyl ether and 40% by weight of butyl acetate obtained by stirring the individual constituents together.

E) Mixture of 55% by weight of dimethylacetamide, 45% by weight of a mixture of 2,6-di-tert-butyl-4-methylphenol and 2,2′-methylenebis(6-tert-butyl-methylphenol) in the ratio 1:1, obtained by stirring the individual constituents together.

F) 30% by weight of tert-butylhydroquinone in 30% by weight of diethylene glycol monobutyl ether, 38% by weight of butyl acetate and 2% by weight of citric acid, obtained by reaching together the individual constituents.

G) Mixture of 45% by weight of 2,2-dimethyl-1,3-dioxolane-4-methanol, 8% by weight of 2-isobutyl-2-methyl-1,3-dioxolane-4-methanol, 45% by weight of a mixture of 2,6-di-tert-butyl-4-methylphenol and 2,2′-methylenebis(6-tert-butyl-4-methylphenol) and tert-butylhydroquinone in the ratio 0.7:3:0.8 and 2% by weight of tolyltriazole obtained by stirring together the individual constituents.

H) Mixture of 45% by weight of 2,2-dimethyl-1,3-dioxolane-4-methanol, 10% by weight of 2-isobutyl-2-methyl-1,3-dioxolane-4-methanol, 45% by weight of a mixture of 2,6-di-tert-butyl-4-methylphenol and 2,2′-methylenebis(6-tart-butyl-4-methylphenol) and tert-butylhydroquinone in the ratio 0.7:3:0.8 obtained by stirring together the individual constituents.

Performing the Rancimat Tests:

To ascertain the oxidation stability, the Rancimat test in accordance with DIN 14112 was used (Rancimat 679 from Metrohm AG). In this, at a temperature of 110° C., air is passed through the sample, and the volatile oxidation products formed following consumption of the antioxidants present (500 or 1000 ppm) are measured via the change in conductivity. This is used to determine the induction time, i.e. the start of oxidation via the tangent method.

The influence of copper was tested as follows: rapeseed or soybean methyl esters were provided with 1000 ppm of antioxidant (mixtures A to H) and tested as to oxidation stability by the Rancimat method (DIN 14112) at 110° C. Here, in each case, a copper strip as described in EN 2160 was hung in the bath.

In this connection, the parameters specified below were determined as follows:

TABLE 1 Product properties, determined in rapeseed methyl ester Product C) E) G) H) According According According According A) B) to the D) to the F) to the to the property Comparison Comparison invention Comparison invention Comparison invention invention Appearance Yellow Yellow Yellow Amber Yellow Amber yellow Yellow Yellow liquid liquid liquid yellow liquid liquid liquid liquid liquid Viscosity 8 14 10 n.d. 10 n.d. n.d. n.d. [mPa · s] Crystallization Yes Yes No No No No No No at −5° C. Flashpoint 147° C. 172° C. 70° C. 38° C. 70° C. 38° C. 92° C. 92° C. Acid number <0.2 mg <0.2 mg <0.2 mg 0.4 mg <0.2 mg 17.5 mg KOH/g <0.2 mg <0.2 mg according to EN KOH/g KOH/g KOH/g KOH/g KOH/g KOH/g KOH/g 14104 Rancimat 6.2 h 7.5 h  9.6 h  9.6 h  9.6 h  9.5 h  9.7 h  9.7 h according to 8.1 h 9.1 h 11.7 h 13.9 h 11.7 h 11.4 h 12.7 h 12.7 h DIN14112 in rapeseed methyl ester at 500 or 1000 ppm* “Corrosion”  <1 h  <1 h  9.3 h   <1 h   <1 h  9.0 h  9.3 h   <1 h (Rancimat with  <1 h  <1 h 11.6 h   <1 h   <1 h 11.7 h 12.5 h   <1 h copper strip) *Starting stability without antioxidant: 5.0 h n.d. = not determined

TABLE 2 Product properties, determined in soybean methyl ester Product C) E) G) H) According According According According A) B) to the D) to the F) to the to the property Comparison Comparison invention Comparison invention Comparison invention invention Appearance Yellow Yellow liquid Yellow Amber Yellow Amber Yellow Yellow liquid liquid yellow liquid liquid yellow liquid liquid liquid Viscosity 8 14 10 14 10 12 n.d. n.d. [mPa · s] Crystallization Yes Yes No No No No No No at −5° C. Flashpoint 147° C. 172° C. 70° C. 38° C. 70° C. 38° C. 92° C. 92° C. Acid number <0.2 mg <0.2 mg <0.2 mg 0.4 mg <0.2 mg 17.5 mg <0.2 mg <0.2 mg according to EN KOH/g KOH/g KOH/g KOH/g KOH/g KOH/g KOH/g KOH/g 14104 Rancimat 6.3 h 7.5 h  9.6 h 14.2 h  9.6 h  9.5 h 11.6 h 11.3 h according to 6.8 h 8.5 h 11.5 h 20.4 h 11.5 h 11.4 h 20.7 h 21.2 h DIN14112 in soybean methyl ester at 500 or 1000 ppm* “Corrosion”  <1 h  <1 h  9.3 h   <1 h   <1 h  9.0 h 11.0 h   <1 h (Rancimat with  <1 h  <1 h 11.6 h   <1 h   <1 h 13.7 h 20.3 h   <1 h copper strip) *Starting stability without antioxidant: 5.5 h

As is evident from the tables, the mixtures produced by the method according to the invention exhibit a high Rancimat value coupled with simultaneously high flashpoint, and no evident corrosion as soon as tolyltriazole was used.

The mixture D listed as comparison example has a comparatively low flashpoint, which considerably hinders handling and dosing. The mixture F listed as comparative example likewise exhibits a low flashpoint and additionally an increased acid number on account of the chelating agent citric acid present herein. This may lead to an increase in corrosion rate. 

1. A method for the production of oxidation-stabilized biodiesel, the method comprising mixing biodiesel with 0.005 to 0.5% by weight of a composition comprising: A) 40-60% by weight of a mixture of at least one C₁-C₄-alkylated mono-phenol and at least one C₁-C₄-alkylated bisphenol, B) 1-5% by weight of a chelating agent for copper and zinc ions, and C) 35-59% by weight of solvent selected from the group of aprotic amides with a chain length of C₂-C₄ and/or cyclic ketals and/or dibasic esters with a chain length of C₄-C₅, where a sum of the constituents A), B) and C) is 100% by weight.
 2. The method as claimed in claim 1, wherein the mixture of at least one C₁-C₄-alkylated mono-phenol and at least one C₁-C₄-alkylated bisphenol is a mixture comprising 2,6-di-tert-butyl-4-methylphenol (BHT) and 2,2′-methylenebis(6-tert-butyl-4-methylphenol).
 3. The method as claimed in claim 2, wherein the chelating agent comprises aromatic triazoles which have a pH>6 in aqueous solution.
 4. The method as claimed in claim 3, wherein the aromatic, triazoles are benzotriazole and/or tolyltriazole.
 5. The method as claimed in claim 1, wherein the solvent is at least one aprotic amide.
 6. The method as claimed in claim 5, wherein the aprotic amide is N,N-dimethylacetamides, N,N-dimethylpropionylamide and/or N-methylpyrrolidone.
 7. The method as claimed in claim 1, wherein the cyclic ketals are the reaction products of glycerol with at least one C₁- to C₆-aldehyde and/or the reaction products of glycerol with at least one C₃- to C₆-ketone.
 8. The method as claimed in claim 7, wherein the cyclic ketals are 2,2-dimethyl-1,3-dioxolane-4-methanol, or a mixture of 2,2-dimethyl-1,3-dioxolane-4-methanol and 2-isobutyl-2-methyl-1,3-dioxolane-4-methanol.
 9. The method as claimed in claim 2, wherein a ratio of 2,6-di-tert-butyl-4-methylphenol (BHT) to 2,2′-methylenebis(6-tert-butyl-4-methylphenol is 1:10 to 2:1.
 10. The method as claimed in claim 1, wherein the component (A) further comprises 1-9% by weight of tert-butylhydroquinone.
 11. The method as claimed in claim 1, wherein the stabilized biodiesel is fuel.
 12. Oxygen-stabilized biodiesel comprising 0.005 to 0.5% by weight of stabilizer comprising: A) 40-60% by weight of a mixture of at least one C₁-C₄-alkylated mono-phenol and at least one C₁-C₄-alkylated bisphenol, B) 0-5% by weight of chelating agent selected from the group of the aromatic triazoles, and C) 35-59% by weight of solvent selected from the group of aprotic amides with a chain length of C₂-C₄ and/or cyclic ketals and/or dibasic esters, where a sum of the constituents A), B) and C) is 100% by weight.
 13. A fuel-stabilizer comprising: A) 40-60% by weight of a mixture of at least one C₁-C₄-alkylated mono-phenol and at least one C₁-C₄-alkylated bisphenol, B) 0-5% by weight of chelating agent for copper and zinc ions, and C) 35-59% by weight of solvent selected from the group of aprotic amides with a chain length of C₄-C₆ and/or cyclic ketals and/or dibasic esters.
 14. The method as claimed in claim 1, wherein the composition comprises: 45-55 wt % of the mixture of at least one C₁-C₄-alkylated mono-phenol and at least one C₁-C₄-alkylated bisphenol; 1.2-2.6% by weight, of the chelating agent, and 35-59% by weight of the solvent
 15. The method as claimed in claim 14, wherein: the mixture of at least one C₁-C₄-alkylated mono-phenol and at least one C₁-C₄-alkylated bisphenol is a mixture comprising 2,6-di-tert-butyl-4-methylphenol (BHT) and 2,2′-methylenebis(6-tert-butyl-4-methylphenol) at a ratio of 1:10 to 2:1; the aromatic triazoles are benzotriazole and/or tolyltriazole; and the solvent is N,N-dimethylacetamides, N,N-dimethylpropionylamide and/or N-methylpyrrolidone.
 16. The oxygen-stabilized biodiesel as claimed in claim 12, wherein the stabilizer comprises: 45-55 wt % of the mixture of at least one C₁-C₄-alkylated mono-phenol and at least one C₁-C₄-alkylated bisphenol; 1.2-2.6% by weight, of the chelating agent, and 35-59% by weight of the solvent.
 17. The oxygen-stabilized biodiesel as claimed in claim 16, wherein: the mixture of at least on C₁-C₄-alkylated mono-phenol and at least one C₁-C₄-alkylated bisphenol is a mixture comprising 2,6-di-tert-butyl-4-methylphenol (BHT) and 2,2′-methylenebis(6-tert-butyl-4-methylphenol) at a ratio of 1:10 to 2:1; the aromatic triazoles are benzotriazole and/or tolyltriazole; and the solvent is N,N-dimethylacetamides, N,N-dimethylpropionylamide and/or N-methylpyrrolidone. 